1. Field of the Invention
The invention relates to a ring oscillator that performs a process for obtaining a variable frequency signal and variable frequency oscillators embodied by means of a loop comprising an odd number of inverting cells connected in series, some of which are variable delay cells. The control of the delay acts directly on the value of the frequency. Cells of this type can be used in other applications such as signal synchronization.
2. Description of Related Art
According to the prior art, a control signal can be used to cause the variation of a leading edge of an input signal to a cell for changing the state of the output signal of the cell at the change of state of the input signal. A steep leading edge makes it possible to obtain a short delay. A leading edge with a gradual slope makes it possible to obtain a longer delay. An oscillating signal is obtained whose semi-oscillation is equal to the sum of the delays of each cell. When it comes to obtaining high frequencies, for example higher than 1 GHz, cells of this type present a drawback, because it is difficult to reduce the number of cells in a loop, while maintaining a frequency variation range which is sufficient to eliminate the problems of manufacturing tolerances and deviations in voltage and temperature. In effect, a minimal number of inverting cells is required to guarantee that the output signal of a cell reaches an extreme low or high level before its input signal is inverted. The minimum period obtained with the addition of the intrinsic delays of each cell therefore limits the maximum frequency that can be obtained.
The object of the invention is to provide a ring oscillator which produces an associated high-frequency oscillation which is variable within an appreciable range.
In order to obtain a variable frequency signal of this type, the invention offers a process which loops an inverted signal to an input of an oscillator with a delay controlled by a control signal, characterized in that an inversion of the inverted signal has a hysteresis with a magnitude that is a function of the control signal.
The invention is also embodied in a variable delay cell wherein the transition time between two states is minimal, whatever the delay controlled within an appreciable range, for example spanning from 1 to 16.
The invention offers a cell for obtaining a signal extracted with a variable delay from an input signal. An inverting amplifier having gain controlled by a control signal is used. Hysteresis is provided. The gain of the amplifier is controlled by feeding back the extracted signal, i.e., the output signal, so as to act on the gain of the amplifier thereby controlling the amplification of the input signal (inp).